Cognitive radio is viewed as a novel approach for improving the utilization of a precious natural resource: the radio electromagnetic spectrum. The cognitive radio, built on a software-defined radio, is defined as an intelligent wireless communication system that is aware of its environment and uses the methodology of understanding-by-building to learn from the environment and adapt to statistical variations in the input stimuli, with two primary objectives in mind: /spl middot/ highly reliable communication whenever and wherever needed; /spl middot/ efficient utilization of the radio spectrum. Following the discussion of interference temperature as a new metric for the quantification and management of interference, the paper addresses three fundamental cognitive tasks. 1) Radio-scene analysis. 2) Channel-state estimation and predictive modeling. 3) Transmit-power control and dynamic spectrum management. This work also discusses the emergent behavior of cognitive radio.

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Cognitive radio: brain-empowered wireless communications

Singular-spectrum analysis: a toolkit for short, noisy chaotic signals

Large amounts of data, like one or more time series from some spacecraft carried instruments, have to be reduced and presented in understandable quantities. As physical theories and models often are formulated in terms of frequency rather than time, it is often useful to transform the data from the time domain into the frequency domain. The transformation to frequency domain and application of statistics on the result is known as spectral analysis. The literature on spectral analysis is voluminous. In most cases, it is written by experts in signal processing, which means that there are many texts available outlining the fundamental mathematics and elaborating the fine points. However, this is not the only background needed for a space physicist who is put to the task of actually analysing spacecraft plasma data. Simple questions on normalisation, physical interpretation, and how to actually use the methods in practical situations are sometimes forgotten in spectral analysis texts. This chapter aims at conveying some information of that sort, offering a complement to the textbooks rather than a substitute for them. The discussion is illustrated with idealised examples as well as real satellite data. In order not to expand the chapter into a book in itself, we concentrate on the application of basic Fourier and wavelet methods, not treating interesting topics in time series analysis like stationarity tests, filtering, correlation functions, and nonlinear analysis methods. Higher order methods like bispectral analysis are also neglected. Fundamentals of such items are covered by many of the references to this chapter, and methods particularly suited for multipoint measurements are of course found throughout this book. Other introductions with multi-spacecraft applications in mind can be found in the paper by Glassmeier and Motschmann [1995] and in other papers in the same publication. The disposition of the chapter is as follows. First, we introduce the basic concepts in Section1.2, where we also discuss time-frequency methods for the analysis of nonstationary signals. The practical implementation of classical Fourier techniques is treated in Section1.3, while the implementation of Morlet wavelet analysis is discussed in Section 1.4. In Section1.5, we turn to the simultaneous analysis of two or more signals by the cross spectrum technique, particularly relevant for the analysis of multipoint measurements. Finally, we touch upon the use of parametric spectral methods in Section 1.6.

Analysis methods for multi-spacecraft data

A general stability analysis is given of the Kevin-Helmholtz instability, for the case of sheared MHD flow of finite thickness in a compressible plasma which allows for the arbitrary orientation of the magnetic field, velocity flow, and wave vector in the plane perpendicular to the velocity gradient. The stability problem is reduced to the solution of a single second-order differential equation including a gravitational term to represent the coupling between the Kelvin-Helmholtz mode and the interchange mode. Compressibility and a magnetic field component parallel to the flow are found to be stabilizing effects, with destabilization of only the fast magnetosonic mode in the transverse case, and the presence of both Alfven and slow magnetosonic components in the parallel case. Analysis results are used in a discussion of the stability of sheared plasma flow at the magnetopause boundary and in the solar wind.

Nonlocal stability analysis of the MHD Kelvin‐Helmholtz instability in a compressible plasma

A general stability analysis is performed for the Kelvin-Helmholtz instability in sheared magnetohydrodynamic flow of finite thickness in a compressible plasma. The analysis allows for arbitrary orientation of the magnetic field B0, velocity flow v0, and wave vector k in the plane perpendicular to the velocity gradient, and no restrictions are imposed on the sound or Alfven Mach numbers. The stability problem is reduced to the solution of a single second-order differential equation, which includes a gravitational term to represent coupling between the Kelvin-Helmholtz mode and the interchange mode. In the incompressible limit it is shown that the Kelvin-Helmholtz mode is completely stabilized for any velocity profile as long as the condition is satisfied, where V0 is the total velocity jump across the shear layer. Numerical results are obtained for a hyperbolic tangent velocity profile for the transverse (B0 ⊥ v0) and parallel (B0∥v0) flow configurations. Only modes with kΔ < 2 are unstable, where Δ is the scale length of the shear layer. The fastest growing modes occur for kΔ ∼ 0.5-1.0. Compressibility and a magnetic field component parallel to the flow are found to be stabilizing effects. For the transverse case, only the fast magnetosonic mode is destabilized, but if k · B0 ≠ 0, the instability contains Alfven-mode and slow-mode components as well. The Alfven component gives rise to a field-aligned current inside the shear layer. In the parallel case, both Alfven and slow magnetosonic components are present, with the Alfven mode confined inside the shear layer. The results of the analysis are used to discuss the stability of sheared plasma flow at the magnetopause boundary and in the solar wind. At the magnetopause boundary, the fastest growing Kelvin-Helmholtz mode has a frequency of 0 (V0/2Δ), which overlaps with the frequency range of geomagnetic pulsations (Pc 3-5). It is suggested that the MHD Kelvin-Helmholtz instability could serve as a dynamo process driving small-scale field-aligned currents in the presence of the sheared plasma flow in the magnetosphere.

Nonlocal stability analysis of the MHD Kelvin-Helmholtz instability in a compressible plasma. [solar wind-magnetosphere interaction]

Time-series analysis of the fluxes of interplanetary charged particles measured by the Ulysses and Voyager spacecraft reveals many periodic components. From 1 to 140 µHz, the spectral components are consistent with those estimated (but not confirmed) for gravity-mode oscillations of the Sun: from 1,000 to 4,000 µHz, the spectral lines closely match the frequencies of known solar pressure modes. These concordances imply that the solar wind and the interplanetary magnetic field transmit solar oscillations and thus might be used to probe the interior structure of the Sun.

Propagation of solar oscillations through the interplanetary medium

Magnetic impulse events were selected by a computer algorithm procedure from magnetic records obtained at the near cusp latitude conjugate stations Iqaluit, Northwest Territories, Canada, and South Pole Station, Antarctica. The algorithm was constructed to select large (≳ 50 nT in the vertical component of the magnetic field), short lived (6 to 12 min) events. These events were found to be highly localized in the 06 to 18 LT sector at the two stations. A strong minimum in occurrence was found during hour 11 LT. The field changes associated with these events can be interpreted as due to an approximately half-cycle, odd-mode, Alfven wave along a near-magnetopause flux tube. From the vertical magnetic deflections of the impulse events the directions of field-aligned currents into the conjugate ionospheres were inferred. In the morning LT sector, field-aligned currents were directed into the ionospheres, while in the afternoon LT sector, field-aligned currents were directed out of the ionospheres. These findings are comparable with the statistical results for quasi-stationary field-aligned currents and suggest that at the times of these events, Iqaluit and South Pole are at a higher effective magnetic latitude. The average deflection in the vertical component for the events was measured to be ∼ 95 nT. From this the magnitude of the average field-aligned currents was calculated to be J∥ ∼ 2 × 10−7 A/m².

Cusp latitude magnetic impulse events: 1. Occurrence statistics

A special campaign was conducted at the Sondre Stromfjord incoherent scatter radar facility in October 1985 to study hydromagnetic and atmospheric gravity were phenomena near the cusp regions of the magnetosphere. During a day when the convection reversal boundary was measured just to the north of the radar, a field-aligned current filament of inferred amplitude --2 x 10V A was measured at the boundary. Both ion and electron heating appeared to accompany the current filament. Magnetic field data acquired in the southern conjugate hemisphere are used to conclude that the current filament was probably on closed field lines. The current filament may be evidence of the magnetosphere boundary layer on closed field lines, perhaps produced by the injection of magnetosheath plasmoids onto the field lines by a flux transfer-type reconnection process. copyright American Geophysical Union

Ionosphere and ground‐based response to field‐aligned currents near the magnetospheric cusp regions

We investigate the relationship of local daytime geomagnetic energy (measured in the period range ∼30–300 s at a mid-latitude ground-based station) to interplanetary parameters during two time intervals when the solar wind/interplanetary field conditions were quite different. We find that the geomagnetic energy levels in the entire period band are consistent with production by a Kelvin-Helmholtz instability source at the earth's magnetopause. We further find that at the shorter periods (∼30–60 s) the energy may also have a source contribution from the mechanism proposed by Greenstadt, which involves wave energy transfer from the earth's bow shock to the magnetopause. This result represents the first evidence of frequency dependencies in the interplanetary source mechanisms for hydromagnetic energy production in the magnetosphere.

Dependence of hydromagnetic energy spectra on solar wind velocity and interplanetary magnetic field direction

Magnetic field data acquired at high-latitude, near-conjugate stations (Iqaluit, Northwest Territories, Canada, and South Pole Station, Antarctica) are studied in order to examine in more detail the nature of magnetic “impulse” signatures that occur in the data and that are produced by ionosphere currents which are caused by magnetopause processes. An examination of the data, both visually and with a computer algorithm “detector,” from the 5-month interval July–November 1985 found many such magnetic “impulse” events which could be interpreted in terms of intense field-aligned currents above the observing stations. All the events have a half-width in time of a few minutes, and most are reasonably conjugate. The majority of the events studied have magnetic field perturbations in the vertical component which can be interpreted in terms of field-aligned currents directed in the same direction (either into or out of the ionosphere) in both hemispheres. The perturbations in the horizontal plane are consistent with an interpretation in terms of a single cycle of an odd mode Alfven wave. For the events shown in detail in this paper, the impulsive magnetic signatures are found to occur for the interplanetary magnetic field BZ component positive, negative, or variable. The observations are discussed in the context of some contemporary ideas on the generation of ionospheric disturbances by magnetopause processes such as sporadic reconnection (“flux transfer events”), plasma injections into the low-latitude boundary layer, Kelvin-Helmholtz instability, and solar wind “pressure pulses.” Near-equatorial data from a location in the same local time section as the high-latitude data are used to show the gross differences in global ionospheric currents stimulated by sudden commencements and by the magnetic impulse events.

L. J. Lanzerotti

Papers

Propagation of solar oscillations through the interplanetary medium

Cusp latitude magnetic impulse events: 1. Occurrence statistics

Ionosphere and ground‐based response to field‐aligned currents near the magnetospheric cusp regions

Dependence of hydromagnetic energy spectra on solar wind velocity and interplanetary magnetic field direction

Magnetic impulse events at high latitudes: Magnetopause and boundary layer plasma processes